The present invention relates to equipment for use in counting, sorting as to size and shape, and displaying in count, size and shape format the particle content of a fluid.
The need exists for apparatus capable of rapid analysis of particles, particularly particles suspended in a moving fluid. Applications for such apparatus are widespread and include such diverse fields as water and air pollution studies, ore refining, bacteriology, pathology, metallurgy, basic medical research and medical diagnosis.
While some applications are accommodated by using laboratory type instrumentation, a great many other applications require instruments which can operate on-line, in real-time so that the information gained from the analysis may be used in a process influencing manner to enhance the yield of the process.
In the past, assessment of the particle content of fluids has been accomplished by preparing for microscopy and analyzing a sample taken under controlled conditions. Photographs of precise enlargement may be taken of the sample and may be visually examined, or direct visual examination of the sample through the microscope may be used as well. A laboratory technician counts the number of particles and classifies them as to size in a representative and precisely known area of the sample. The process is tedious, time consuming and subject to wide variation according to the skill, acuity of vision and morale of the technician. These problems are further compounded when particles of a great range of sizes and shapes are present in the sample.
To overcome these disadvantages, systems have been devised which employ analog television cameras or discreet beam spot scanners both of which employ the principle of dividing a fixed field of view into a number of adjacent scan lines each of which represents a narrow slice of the field of view. By electronic timing and synchronization, the presence and location of particles in each line may be detected. Adjacent lines may then be electronically compared and the presence or absence of the image on each adjacent line determined. By measuring the time each particle is present in a single scan and by counting the number of successive scans in which the particle is present, a two-dimensional measurement of the particle is accomplished.
While a motionless sample may be scanned in this manner without difficulty, if the sample is in motion, as will occur for example in almost all process control applications, it will be appreciated that the problems of rendering unambiguous counts of particles and of determining the sizes and shapes of particles are increased considerably. Conventional television cameras typically have frame rates of 10 to 30 frames per second, and lines per frame of from 200 to 1000. As sample flow velocities become low with respect to the rate at which the line scan moves across the field, the tendency for duplicate counts to occur is increased. As the sample flow velocity becomes high and reaches an appreciable fraction of the line scan rate, the tendency to miss counts becomes greater. Furthermore, the counts of the smaller particles in a grouping will be missed more frequently than will be counts of the larger particles due to the lower probability of detection for the smaller particles. While rates of flow may be adjusted to within a narrow range by, for example, controlling dilution of the main sample, the ambiguity cannot be completely resolved. Furthermore, the skew in data which results from a varying probability of detection cannot be easily corrected by statistical manipulation since the particle distribution functions are typically rather irregular. Crude simplifying assumptions as to the behavior of the curve of the distribution function are required in order to make even rudimentary corrections.
Shape detection is also adversely affected by flowing samples. If the particle image velocity is such that its image crosses the image plane in the same direction and in the same time as the scan, the particle image will appear to be as long as the screen is high. An opposite effect, i.e. apparent shortening of the image, will occur if the motion is in an opposite direction. Using stroboscopic illumination of less than the frame completion time is of course impossible since only a portion of the scan would be illuminated. Therefore, illumination requirements and frame rates are always interrelated in analog television cameras. It will be readily apparent from this that "motion stopping" action by means of stroboscopic illumination is limited to particle velocities which traverse the field of view in times much greater than the time required for a complete frame scan.
While it might seem that increasing or decreasing scan rates would be desirable in order to accommodate increases or decreases in sample flow rates, the basic scheme of operation of analog television cameras precludes variation of scan rates in practical systems. This practical limitation results from the need for critical timing and synchronization of the system. To make useable the video output of the camera, the location of the scanning beam at which each increment of video output is produced must be precisely known. In order to know precisely this location, the exact timing of the beginning of the scan, the exact rate of the scan, and the exact position of each scanning line on the camera screen must be precisely known and controlled. In practical systems, the timing and scan rates are rigidly fixed and every effort is made to prevent them from varying. While it is theoretically possible to make such a system operate asynchronously, from a practical standpont this is not a sound approach since extreme complication in the design of all aspects of the system is the immediate result of attempts to make the system operate asynchronously.
An additional problem in analog television system is that the sensitivity of the system is related to the rate at which the beam sweeps the screen. Slower sweeps may be used to increase the sensitivity, but this reduces the maximum sample rate and limits the speed of sample flow which can be accommodated. Conversely, sample rates may be increased but at the expense of sensitivity. Thus, if either parameter is optimized, the other suffers proportionally.
Some of the sensitivity limitation may be overcome by using higher illumination levels when higher than normal frame rates are employed. A practical limitation on maximum useable illumination is quickly reached, however, since high intensity illumination brings into play size, weight and thermal dissipation considerations which make compact sample chamber designs impractical.
Errors are common to all scanning methods of particle analysis due to factors such as coincidence of particles, irregular shapes of particles and edge-scanning of particles. The customary method for handling these problems is to make a statistical adjustment in the output data. This method is usually satisfactory depending upon the precision required for a particular application. A more vexing problem, however, is the error due to the presence of bubbles in the sample. Bubbles may be present in all size ranges and are a particular concern when the sample is drawn from highly turbulent processes. A ball mill cyclone in the ore refining industry is a good example of such a sample source.
The bubble presents an unusually troublesome source of errors because its optical image appears essentially the same as a solid, spherical particle. The quantity of bubbles present in a sample is unpredictable and therefore statistical corrections are impossible. The analyzer must therefore be capable of distinguishing bubbles from solid particles in order to yield an accurate count. Some systems have attempted to avoid this problem by eliminating bubbles from the sample fluid. This approach is technically sound, but the apparatus which is required is bulky, heavy, and mechanically complex.
One property of the optical image of a bubble allows it to be distinguished from that of a solid particle if apparatus is available which is capable of pattern recognition. Most of the light which is incident upon the surface of a bubble is refracted away from its original path, leaving an apparent shadow, as would a solid particle. However, at the center of the bubble, light is perpendicular to the surfaces of the bubble and is therefore not refracted. Instead it passes through the bubble, giving the image of the bubble the appearance of a solid particle having a small aperture at the center.
Systems which rely on edge detection by a scanning beam are confounded by this shape. Since simple analog scanning systems do not store image data, it is impossible to perform even simple pattern recognition routines without resorting to elaborate analog storage and retrieval systems. On-line operation of such a system would be prohibitively slow, using known techniques, and would require conversion of analog data to a digital format for processing, thereby adding to the expense and complexity of the system.